The subunits of Actinomyces viscosus T14V type 1 and Actinomyces naeslundii WVU45 type 2 fimbriae have been identified by cloning and expression of the respective genes in EScherichia coli. The amino acid sequence of each subunit was deduced from the nucleotide sequence of the cloned gene and the N-terminal sequence confirmed by Edman degradation of purified fimbriae and cloned fimbrial subunit. The genes for type 1 and type 2 fimbrial subunits encode proteins of approximately 530 amino acid residues of which the first 30 to 32 appear to be signal sequences. An evolutionary relationship between the subunits of Actinomyces spp. type 1 and type 2 fimbriae was evident from amino acid sequence homologies. Studies of bacterial adherence using A. viscosus T14V and mutants specifically lacking type 1, type 2 or both types of fimbriae have provided evidence that the acidic proline rich proteins are specific receptors for type fimbriae. These salivary components are not glycosylated and thus, their recognition by A. viscosus type 1 fimbriae most likely depends on protein-protein interactions. In contrast, the receptor molecule on Streptococcus sanguis 34 for Actinomyces spp. type 2 fimbriae is a linear polysaccharide composed of phosphodiester-linked hexasaccharide subunits. The reaction of rabbit antibody with this polysaccharide depends on the nonreducing end of the hexasaccharide chain while GalNAc beta 1 leads 3Gal at the reducing end is involved in lectin recognition. A similar separation of the antigenic determinant and receptor structure occurs within the polysaccharide of S. sanguis J22. However, with strain J22, Gal beta 1 leads to 3GalNAc at the reducing end of the oligosaccharide subunit appears to be involved in lectin recognition. Thus, the receptor activity of these streptococcal polysaccharides may depend on internal structural features that mimic the glycoprotein and glycolipid receptors on host cells.